Although it was recognised in the nineteenth century that a selective gas separation can be performed as a result of different permeation velocities of the respective components, it was not until the 1960s that suitable membranes with adequately high permeability and separation efficiencies became available to allow such gas separation to be performed on a significant commercial scale. Since the 1960s the use of semi-permeable membranes to separate gas mixtures has become a well known technique in the production of industrial gases. Known plants for the separation of a gas mixture by membranes are constructed so as to present a large surface area of membrane to the gas mixture to be separated. For example, such plants may employ a multitude of identical, elongate, hollow fibres which are formed of a suitable semi-permeable membrane and which extend in parallel to one another. The fibres are appropriately mounted in a pressure vessel. The gas mixture to be separated is fed into the pressure vessel at or near one end outside the fibres. It flows longitudinally of the fibres. The insides of the fibres are maintained at a pressure lower than that which obtains on the outside of them. If the gas mixture to be separated consists of two components, the faster permeating component passes more and more to the low pressure side. Accordingly the gas on the outside of the fibres (the high pressure side) becomes richer in the slower permeating component as it flows along the outside of the fibres and a product gas, rich in the more slowly diffusing component, may be withdrawn at pressure from the end of the pressure vessel opposite that at which the feed gas is introduced.
The permeate gas is richer than the feed gas in the faster diffusing component. The permeate gas is withdrawn from the insides of the fibres at the same end of the pressure vessel as that at which the feed gas is introduced. Accordingly, the permeate gas flows generally countercurrently to the feed gas. At any point along a fibre the net rate of diffusion of each component across the membrane depends on the ratio of the partial pressures of that component on the respective sides of the membrane. As the feed gas passes along the membrane, the partial pressure of the more rapidly diffusing component of the mixture falls. We have found that for a given gas inlet and permeate outlet pressure and for a given permeability ratio and cell of given dimensions, there is a limit to the amount to which the more rapidly diffusing component can be reduced in the feed gas when using countercurrent flow.
It is an aim of the present invention to provide a gas separation method and apparatus which reduces or overcomes this difficulty.